There exist many applications for laser diodes, especially laser diode arrays, that can simultaneously or selectively emit separable light from different array elements. Examples are color printing, full color digital film, recording, color displays, and optical recording and playback systems. Building the laser array into a monolithic structure in which the emitting regions are closely spaced offers the further important advantage that all of the optical sources can share a common lens system. It is also important in a number of these applications that the emitting regions in the monolithic structure are individually addressable, and it is also important to be able to individually detect such closely-spaced beams and process any information contained as a result of beam modulation at the source or by reflection from or transmission through optical media.
There are several ways to generate from a laser diode light beams with unique characteristics that will make the beams easier to separate or detect. One way is to change its polarization; a second way is to change its wavelength. Each of the applications referenced above describe either arrays of multi-wavelength laser diodes, or arrays of dual-polarization diodes.
The first above-referenced related application, Ser. No. 07/994,029, whose contents are herein incorporated by reference, describes a structure for generating multiple wavelength or orthogonally polarized beams from a monolithic, addressable, laser diode chip. The structure comprises a stack of epitaxially-deposited layers including multiple, vertically-stacked, quantum well (QW) heterostructures of the same or different compositions, and capable of oscillating at different wavelengths or orthogonal polarizations. The QW structures may be at different levels. By selective etching and/or diffusion techniques, laterally-spaced QW structures can be isolated and separately activated for individual addressing.
The third and fourth above-referenced applications, Ser. Nos. 07/948,524 and 07/948,522, whose contents are herein incorporated by reference, describe the construction and method of manufacture on a common substrate of individually addressable QW lasers that can be caused to oscillate in the transverse electric (TE) or in the transverse magnetic (TM) mode, and QW laser constructions that can be switched from oscillating in the TE polarization mode to the TM polarization mode, or vice versa. This has been accomplished in certain material systems by controlling the type of strain induced in an epitaxially deposited active region due to lattice mismatches with the substrate. Thus, in most material systems allowing heavy hole and light hole transitions, when the n=1 heavy hole is the lowest energy state and therefore the state whose population is most easily inverted, usually true for unstrained and compressively strained III-V alloy systems, TE polarization gain will predominate. However, by reversing the light hole and heavy hole band edges, achieved in certain material systems by inducing tensile strain into the active region, TM polarization gain will predominate. In the degenerate condition, where the light hole and heavy hole bands are substantially coincident in energy, the polarization of the emission can be determined by threshold carrier density and other factors, such as temperature, facet reflectivity, cavity length and intracavity optical loss.
In general, the desired result of controlling polarization can be achieved with either a single QW, carefully adjusted, or separate Qws for TE and TM mode gain, respectively, with the polarization mode of laser oscillation dependent upon the gain characteristic and the threshold gain. The necessary gain characteristic has one polarization with lowest transparency current, and the orthogonal polarization with a greater peak gain. For some range of active region parameters (thickness, composition, placement within the confining regions, etc.), these characteristics can be obtained, and so the polarization will be determined by the threshold gain. Therefore, the polarization of each device can be selected, for example by introducing an additional loss into one of the devices, thereby forcing it to oscillate in the higher-gain polarization. On the other hand, a device without this additional loss will simply oscillate in the polarization which has lowest transparency current. The additional loss could be an unpumped section along the cavity, low mirror reflectivity, shorter cavity, etc. Similarly, the polarization of each device could be switched, by using an intracavity loss modulator. This polarization selectivity mechanism is demonstrated by the polarization-dependent gain-current characteristics shown in FIG. 1 in which threshold gain, g.sub.th, for both TM and TE modes is plotted along the ordinate, and threshold current, I.sub.th, for both modes is plotted along the abscissa. The curve 10 labelled TE shows the gain characteristic for the TE-mode, and the curve 11 labelled TM that for the TM-mode. When the operating conditions are represented by the vertical line 13, to the left of the crossover 14, then the TE-gain is higher and TE-polarized light is emitted. When the operating conditions are represented by the vertical line 15, to the right of the crossover 14, then the TM-gain is higher and TM-polarized light is emitted.
The fifth referenced prior application, Ser. No. 07/949,452, whose contents are herein incorporated by reference, describes a laser diode array comprising a monolithic body with heterostructures comprising a composite active region made up of two or more adjacent QW layers of dissimilar materials. The composite active region is partially disordered by an impurity free interdiffusion of atomic constituents of the QW layers so as to shift the emission wavelength of the active region. By disordering different areas of the active region to different degrees, the different areas will lase at different wavelengths. Distributed Bragg Reflector (DBR) gratings may be added to enhance desired emission wavelengths. As an alternative, a ferroelectric layer structure may be added to double the frequency of emitted light from selected regions of the array.
The third, fourth and fifth related applications specifically describe only dual polarization or multiple wavelength laser diode arrays, but no monolithic structure capable of selectively producing four or more separable emissions.
The first related application also describes a monolithic structure with either multiple wavelength emissions or dual polarized emissions, and also suggests their combination in a single structure. However, no examples are given of how such a structure can be achieved.